1. Field of the Invention
The present invention relates to a semiconductor laser that oscillates at a wavelength in a visible region of green color and yellow color and that is applied to displays, etc. More specifically the invention relates to a semiconductor laser designed in a stacked structure for improving operation characteristics of a semiconductor laser using a compound semiconductor comprising a group II element and a group VI element formed on an InP substrate.
2. Description of the Related Art
A semiconductor device emitting light in visible to ultraviolet regions, that is, a semiconductor laser or a light emitting diode is one of important semiconductor devices in the current society/industrial field, for example, in the applications to optical information recording apparatuses (compact disk(CD), digital versatile disc(DVD), Blu-ray Disc(BD)), light sources for color displays, semiconductor laser excitation, fabrication, sensors, measuring apparatuses, medical use, or white lamps. The outline of the wavelength and semiconductor materials of optical devices is as shown in Table 1.
TABLE 1Semiconductordevice emitting lightfrom yellow toBlue-light devicegreenRed-light deviceInfrared-light device400 nm band500 nm band600 nm band780 nm, 808 nm, 860(particularly 400 to(particularly 635 tonm, 915 nm, 980 nm480 nm)670 nm)bandGroup III-V nitrideNo appropriateGroup III-VGroup III-Vcompoundcompoundcompoundcompoundsemiconductorsemiconductorsemiconductorcomprisingcomprisingcapable ofcomprisingAlGa(In)AsAlGaInNcontinuous-waveAlGaInPoscillation
As can be seen from Table 1, for semiconductor devices that emit light from yellow to green colors in a 500 nm band which is a wavelength band between red and blue colors, even development for materials has not yet been conducted, not to mention the research and development thereof. Accordingly, performance capable of coping with practical use has not yet been attained for semiconductor devices using the wavelength band described above, particularly, semiconductor lasers.
As the semiconductor for optical devices, group II-VI semiconductors are useful along with group III-V compound semiconductors. However, no green-yellow light semiconductor lasers have yet been put to practical use. The reason is mainly attributable to the fact that no sufficient reliability is obtainable. One of the causes for inducing deterioration of devices is multiplication of crystal defects which are micro defects in active layer portions of the laser.
Kato, et al. have made an effort to decrease defects of a stacked layer of ZnSe series materials formed on a GaAs substrate by devising crystal growth conditions, experimentally manufactured a laser having no defects in the active layer region and observed a device life up to 400 hours (Non-Patent Document 1: E. Kato, et al., Electron. Lett. 34, 282 (1998)). It has been pointed out that the life was restricted by movement of micro defects due to compressive strains and micro defects such as nitrogen doping in a P-type cladding layer.
In recent years, studies have been started for group II-V compound semiconductor materials containing beryllium for the group II element as the constituent material of green-yellow semiconductor lasers (Patent Document 1: JP No. 2586349, Patent Document 2: JW-A No. 2000-500288, Patent Document 3: JP-A No. 2004-95922, Non-Patent Document 2: A. Waag et al., Journal of Crystal Growth 184/185(1998)1-10). The present inventors experimentally manufactured BeZnSeTe series LED (Light Emitting Diode) using group II-VI compound semiconductor materials containing beryllium and confirmed a device life of 5000 hours, at room temperature, with an emission wavelength of 570 nm, with an injection current density of 130 A/cm2 (Non-Patent Document 3: Kishino et al., Phys. Stat. Sol., 6, (2004) 1477-1486, Non-Patent Document 4: Hayami, et al., Pretext of the 52th Meeting of Japan Society of Applied Physics, 31p Z-N6, Non-Patent Document 5: Yuki Nakai et al., Phys. Stat. Sol. (a), 201, 12 (2004) pp. 2708 to 2711). The result is considered to provide a progress of the practical use of green-yellow semiconductor lasers.
It is considered that the improvement for the reliability in this system is attributable to the fact that lattice-matched crystal layer can be used by the use of an InP substrate and to the effect of suppressing degradation due to crystal defects and dislocations since the crystal becomes strong due to the introduction of beryllium (Be). Further, a semiconductor laser with a material configuration similar to that of the LED described above was experimentally manufactured and laser oscillation obtained by pulse driving at 77K was confirmed (Non-Patent Document 3). However, a continuous-wave room temperature oscillation required for practical use could not be attained in this structure.
While the BeZnSeTe series semiconductor laser manufactured on the InP substrate has a possibility of greatly improving the reliability compared with existent group II-VI lasers, it has not yet attained continuous-wave room temperature oscillation as the basic characteristic. The present invention intends to solve the problem described above and provide a laser structure capable of attaining high reliability and continuous-wave room temperature laser oscillation necessary for practical use. It also intends to provide a laser structure with easy crystal growth and improvement of yield in view of manufacture of the device. At first, problems in the conventional art described above are shown.
FIGS. 1A and 1B show the structure of a BeZnSeTe series LED prepared on an InP substrate as the conventional art (Kishino et al., Phys. Stat. Sol., 6, (2004) 1477 to 1486, Hayami, et al., Pretext of the 52th meeting of the Japan Society of Applied Physics, 31p-Z-N6, or Yuki Nakai, et al,, Phys. Stat. Sol. (a). 201, 12(2004) pp. 2708 to 2711). FIG. 1A shows a layer structure and FIG. 1B shows a band line-up schematically. The symbols, C. B. and V. B. respectively show a conduction band and a valance electron band. Bold solid lines in FIG. 1B show mini-bands formed in the superlattice. Reference numeral 1 denotes an n-electrode comprising Au and Ge; 2, an InP substrate; 3, an InGaAs buffer layer; 4, a Cl-doped ZnCdSe buffer layer; 5, a lower cladding layer comprising a Cl-doped MgSe/ZnCdSe superlattice; 6, a thick stepped superlattice layer comprising MgSe/ZnCdSe superlattice; 7, a BeZnSeTe active layer; 8, a MgSe/ZnCdSe superlattice layer; 9, an upper cladding layer comprising N-coped MgSe/BeZnTe superlattice; 10, an N-doped BeZnTe/ZnTe superlattice layer; 11, an N-doped ZnTe cap layer; 12, a p-electrode comprising Au; and 13, an insulative layer.
That is, the structure shown in FIGS. 1A and 1B has, as a basic structure, a three layered structure, in which an MgSe/Zn0.48Cd0.52Se (2 ML (monolayer: molecular layer/4ML) n-cladding layer 5 (800 nm thickness) and an MgSe/Be0.48Zn0.52Te (4 ML/6 ML) p-cladding layer 9 (500 nm thickness) are arranged with the BexZn1-xSeyTe1-Y active layer 7 (7.5 nm thickness) as the center. The energy at the conduction band (C.B) edge of the n-cladding layer is lower than that at the conduction band edge of the active layer 7, and both of them are in a so-called type II junction.
Accordingly, in the case of directly connecting the n-cladding layer 5 and the active layer 7, electrons can not be injected sufficiently into the active layer. Accordingly, an MgSe/Zn0.48Cd0.52Se (2 ML/3 ML, 15 nm thickness+2 ML/1 ML, 15 nm thickness) superlattice layer 6 (30 nm thickness), which is obtained by changing the thickness of Zn0.48Cd0.52Se in a stepwise manner, is disposed between the n-cladding layer 5 and the active layer 7. Further, an MgSe/Zn0.48Cd0.52Se (2 ML/1 ML) superlattice layer 8 (5 nm thickness) was disposed between the active layer 7 and the p-cladding layer 9. This is for preventing the degradation of the active layer during interruption of the growth in the crystal growth step. The n-In0.48Ga0.52As buffer layer 3 (200 nm thickness) and the n-Zn0.48Cd0.52Se buffer layer 4 (200 nm thickness) were disposed just above the InP substrate 2, and the MgSe/Be0.48Zn0.52Te superlattice layer (50 nm thickness) and the ZnTe cap layer 10 (5 nm thickness) were disposed above the p-cladding layer 9. They were substantially lattice matched with the InP substrate 2 except for the ZnTe cap layer 10.
It was investigated for the cause that the continuous-wave oscillation at room temperature could not be attained in the laser structure, although the LED operation for the long time of 5000 hrs was confirmed in the structure shown in FIGS. 1A and 1B. A sample in which the composition of the active layer 7 was changed in a range of: 0.08<X<0.19 (0.16<Y<0.32) was manufactured, and photoluminescence and photo-current measurement was conducted. As a result, it was found that type II light emission at the boundary between the active layer 7 and the n-side stepped superlattice layer 6, that is, light emission between the spatially separated conduction band and the valence electron band of the hetero junction portion was caused, which lowered the light emission efficiency in the active layer 7. To prevent the type II light emission, the thick stepped superlattice layer 6 was introduced. However, the effect was not yet sufficient and the type II light emission was remarkable at the beryllium composition of 0.08 or more. Further, from the optical guide wave calculation, it was found that the optical confinement factor to the active layer 7 is as less as about 2%, which results in increase of the threshold value in the laser oscillation. Details for the result of calculation are to be described below.